Several trends exist, today, in the semiconductor device fabrication industry and the electronics industry. Devices are continuously getting smaller and smaller and requiring less and less power. A reason for this is that more personal devices are being fabricated which are very small and portable, thereby relying on a small battery as its supply source. For example, cellular phones, personal computing devices, and personal sound systems are devices in great demand in the consumer market. In addition to being smaller and more portable, personal devices are requiring more computational power and on-chip memory. In light of all these trends, there is a need in the industry to provide a computational device that has a fair amount of memory and logic functions integrated onto the same semiconductor chip. Preferably, this memory will be configured such that if the battery dies, the contents of the memory will be retained. Such a memory device that retains its contents while a signal is not continuously applied to it is called a non-volatile memory. Examples of conventional non-volatile memory include: electrically erasable, programmable read only memory (“EEPROM”) and FLASH EEPROM.
Ferroelectric memory devices, such as the FeRAM devices provide non-volatile data storage through the use of a ferroelectric dielectric material that may be polarized in one direction or another in order to store a binary value. The ferroelectric effect allows for the retention of a stable polarization in the absence of an applied electric field due to the alignment of internal dipoles within the Perovskite crystals in the dielectric material. This alignment may be selectively achieved by application of an electric field that exceeds the coercive field of the material. Conversely, reversal of the applied field reverses the internal dipoles.
Data stored in a ferroelectric memory cell is read by applying an electric field to the cell capacitor. If the field is applied in a direction to switch the internal dipoles, more charge will be moved than if the dipoles are not reversed. As a result, sense amplifiers can measure the charge applied to the cell bit lines and provide an indication of a stored logic “1” or “0” at the relevant output. In such case, a read operation is a destructive operation, and the correct data is then restored to the cell during a precharge operation.
In a simple write operation, an electric field is applied to the cell capacitor to polarize it to the desired state. Briefly, the conventional write mechanism for an FeRAM cell includes inverting the dipoles in the ferroelectric capacitor and holding the state with a potential greater than the coercive voltage for a nominal time period.
Certain ferroelectric dielectrics, such as lead zirconate titanate (PZT) may be produced with a variety of crystallographic textures, including <100>, <111>, <001>, and random textures. For example, polycrystalline PZT films having the {100} crystal planes parallel to the substrate surface are said to be <100> textured. Films composed of crystallites that, on average, have no specific crystallographic orientation relative to the substrate surface are said to be randomly oriented.
Textured PZT having a (111) crystallographic orientation is believed to improve FeRAM bit distribution characteristics. However, depending on the type of the FeRAM architecture and processing methodology employed, forming such a textured PZT ferroelectric dielectric layer has proven challenging.